1. Field of the Invention
The invention relates to an arrangement for contactlessly determining a revolution rate and direction of rotation of a component that rotates during its operation, in particular a magnetizable shaft of a motor vehicle.
2. Related Art
Different revolution rate sensors for determining the revolution rate of a component that rotates during its operation are known from the prior art. Such revolution rate sensors are, for example, used for detecting a revolution rate and a direction of rotation of a gear wheel or a transmission shaft in a gearbox of a motor vehicle.
For contactlessly detecting a revolution rate, revolution rate sensors are usually used that are equipped with a magnetic field sensor (for example a Hall sensor). In such sensors, a magnetic field, which changes when a tooth of the gear wheel passes, is generated by a magnet in the vicinity of the Hall sensor. During this the Hall sensor first detects the so-called background field of the magnet. A field strength arising from the mass of the gear wheel or of a comparable component is added to the field strength of the background field. If a tooth instead of a tooth gap is associated with the sensor, this results in a repeated boosting of the magnetic field for the duration of the association of the tooth with the sensor. Thus if the gear wheel is turning, a fluctuation of the field strength takes place depending on the passage of the teeth and tooth gaps past the sensor. The Hall sensors thus detect the change of the magnetic field and generate electrical signals, from which the revolution rate or a position angle of the gear wheel can be determined. Each tooth thus produces a pulse, and the revolution rate can be determined by counting the pulses. More precisely, the revolution rate sensor does not detect the tooth as such, but detects the transition from tooth to tooth gap or vice-versa.
In particular, revolution rate sensors that comprise two Hall elements are known from the published patent applications DE 41 1 958 A1 and EP 0 800 087 A2. For the measurement of revolution rates, the revolution rate sensor is, for example, disposed opposite the gear wheel such that a tooth of the gear wheel first passes the first Hall sensor and then the second Hall sensor during a rotational movement. For this purpose, the two Hall elements must be disposed offset when viewed in the circumferential direction or the direction of rotation of the gear wheel or of the component. With Hall elements offset in this way when viewed in the direction of rotation of the gear wheel, the tooth first comes to one Hall element and, after a certain angle of rotation, to the second Hall element. The oscillations of the magnetic field of the two sensors resulting therefrom are offset relative to each other when viewed over time. In this manner, not only the angular speed and angular acceleration can be determined, but also the direction of rotation, because depending on the direction of rotation the one Hall element or the other Hall element measures the fluctuation first.
If, however, a rotation sensor is installed such that a stationary tooth is opposite both Hall elements at the same time, i.e., both Hall elements are at the same distance from the edge of the next tooth, then the total magnetic field strength on the two Hall elements is the same at any measurement point in time, and a direction of rotation can no longer be determined. This problem can in particular occur with revolution rate sensors with screw mountings. If the rotary position of the sensor in the screwed-in position is such that the two Hall elements are essentially at the same distance from the edge of the next tooth, the direction of rotation cannot be determined or measurement inaccuracies can occur if the difference in distance is too small or is unknown. With such sensors it must be ensured during installation that the two Hall elements are correctly aligned in the final position, therefore a time-consuming readjustment may be necessary.
In order to avoid this problem, in DE 41 41 958 A1 a revolution rate sensor is proposed with which the two Hall elements and the magnet in the revolution rate sensors are disposed so as to be displaceable relative to each other. For example, it is proposed that the magnet is inserted into a holder that can be displaced relative to the body in the interior of the revolution rate sensor. This does enable incorrect alignments to be corrected, but this is time-consuming, is liable to errors and requires suitably trained skilled staff during the installation.